Thursday, February 19, 2009

Ecology of Quaking Aspen in our Yard

The quaking aspen (Populus tremuloides) performs diverse and intricate ecological roles in our garden and might be the most important plant for wildlife in our yard. In the wild, quaking aspen occur in a wide variety of habitats ranging from riparian areas to dry hillsides, and in the garden, aspen can be planted in a variety of locations especially if it receives partial shade or water. In our yard, they thrive in the absence of supplemental water- in its place we planted them where they will receive some shade.

In the wild, quaking aspen provide important breeding, nesting and cover habitat, and forage for a variety of birds and mammals. Quaking aspen is host to a variety of insects, some of which are important foods for other animals including woodpeckers. Wildlife species consume nearly every part of aspen at one time of the year ranging from the roots, to shoots to bark and catkins. Because of this, in the wild, it is truly a keystone species in many habitats.

Quaking aspen are native to the Missoula area, though they are probably at the edge of their range. This location can cause stress and make them susceptible to a variety of pests.

Though a beautiful tree, quaking aspen are maligned by many urban foresters because of their tendency to sucker, and for their predisposition to borer beetle infestations. However, borers are just one of the many insects that use the aspen.

Here is how it often goes…In our yard, it might begin with the bald-faced hornet looking for nesting material. The hornets chew the aspen bark, and mix it with their saliva to create their papery nests. The holes they create in the aspen bark allow the aspen borer beetle (see photo above), to easily open up the cambium to deposit the eggs (more on this later…). Bald-faced hornets are big and mean looking, and often mistaken for yellow jackets. Bald-faced hornets are truly beneficial in your garden, and they even pollinate flowers as summer goes on. One of their main prey is yellow jackets and they even lie in wait for them near food sources. You have probably noticed the two species near a food source, but, watch closely, and you may see the bald-faced hornets ambush and kill yellow jackets. Frankly, I am not a huge fan of yellow jackets, so this is pretty neat and gratifying to see.
Back to the borers. Once the adult borer lays its eggs in the bark of the aspen (see photo above, and notice all the pock marks on the aspen bark from bald-face hornet gnawing), the cool stuff really starts to happen. The aspen borers are a type of long-horned beetle (named for the very long antennae in adults- not for horns, unfortunately), and the one we have in our yard is Saperda calcarata, (also called Poplar borer). Some of these insects may reach two inches long, and are very well camouflaged mimicking the texture and colors of aspen bark and fungi (see photo below). The entire life cycle (from egg to adult) takes about three years in our area.
The borer's eggs typically hatch within two weeks, and the newly emerged larvae chew through the bark, where they spend the first year. As the larvae grow, they eat and tunnel through the wood deeper into the heartwood and sapwood for two years. This tunneling produces the bulges and lumps you see in the trunk. All the while, they force coarse shavings, like sawdust out the holes. In the summer, you can literally watch this happen – just look for fresh sawdust at the base of an aspen at follow it up and you will see saw dust emerging from holes in the trunk. If you look closely enough, you may even see the larvae. The chewing and tunneling weakens the tree and allows the invasion of canker and fungi.

The accumulation of saw dust and borings beneath the aspen is home to the chrysalis of several moth species in our yard including the five-lined sphinx moth. Whereas butterfly caterpillars often pupate in a chrysalis that is suspended beneath a leaf or twig, sphinx moth caterpillars burrow into loose, dry duff to pupate. The loose, dry wood shavings the borer larvae produce is perfect for sphinx moth caterpillars to excavated and pupate within. The chrysalises are dark brown to almost black and look nearly dead. If you do find one when you are rooting around, if you watch carefully you might be able to see it move. Just place it back into the ground.

The five-lined sphinx moth, or commonly called a hummingbird moth, caterpillars are specific hosts of the Oenothera flava (yellow evening primrose) in our yard. The giant caterpillars have marking on them that mimic the shape of the narrow, serrated leaves- fantastically camouflaging the 4” caterpillars (see photo below).
As a result of the larvae boring though the tree, the holes “bleed” or produce sap you can see running down the sides of the aspen. This sap is a great food source for a variety of insects, including butterflies. In our yard, the mourning cloak (Nymphalis antiopa, the Montanan state butterfly) is especially fond of it, and sap is its primary food. The mourning cloak only lays its eggs on twigs or in the shallow cavities of the quaking aspen, or related species, where the adults and caterpillars feed. In addition to butterflies, the sap attracts a variety of other insects from ants to yellow jackets and ultimately bald-faced hornets, which prey on the insects, and the cycle continues.

The presence of the aspen borer in the tree further weakens the aspen making it susceptible to more invasions of aspen beetles. These stresses on the tree also cause it to sucker- which is good because at this point the aspen is on its way to dying (though it make take 6-20 years for the initial trunk to truly die and by then a sucker is there to replace it). Quaking aspen seldom occur individually, and having a cluster of them is not only more natural and more beautiful looking, but it is also a way to provide for its longevity and succession.

In the fall, after about two years of growth, the larvae will move shallower- just beneath the bark, where they pupate during the winter. In February, in our yard, downy woodpeckers will drill into the bark looking for these pupae. In doing so, the woodpeckers drill holes that will create small cavities for insects to hide, lay eggs and even for adult aspen borers to deposit the eggs of a new generation. Once fully developed, the adult aspen borer beetles emerge through a hole in the bark, and spends its adult life mating, and dispersing is eggs to host quaking aspen.

The cavities and holes left by the borers create wonderful places for a variety of insects and spiders to raise their young. In turn, insect eating birds, like warblers, chickadees and others scour the aspens gleaning insects, spider eggs and others from the trunk, crevices and cavities of the aspen.

All this can happen around a quaking aspen in your yard, or, you could use a few types of insecticide and kill the borers.


  1. Yep, I see what you're saying. Last summer there was an infestation of some carterpillars that struck many of the native oaks growing on Stanford campus. People wined, moaned, and complained. Wisely, the landscaping staff left things well alone. The oaks came through just fine, and I'm sure the birds got fat and happy.

  2. I have a somewhat similar story, David. Several years ago, there was a bad outbreak of viburnum leaf beetle, which can reduce the foliage a V. trilobum or other species to ribbons in very little time. I noticed I had them in my highbush cranberry, and I was trying to decide whether or not to get some B. t. to treat the shrub with. However, while I was mulling for a day or two, I walked out in the garden late one afternoon to find the shrub full of waxwings, apparently snacking on the beetle larvae. Problem solved. That same summer we also had a bad session with the white tussock moth larvae, intent on eating just about everything in sight. My husband was having fifteen fits about his horse chestnut being dined on, but then I showed him that we had monarch caterpillars in our asclepias. He agreed that even using an organic solution wouldn't be a solution if it meant risking the monarchs. I promised his tree (and other plants) would be just fine. They were. (of course)
    If we'd just remember to be a bit more laissez faire in our gardening! It's easy for me, in a rural area, but for those caught up in the traumas of suburbia and worrying about having the 'perfect' yard, I guess it can be a challenge. One more good reason not to live in the burbs, yes?

  3. I always meant to plant some aspen by that little red house, but didn't know what a complex sequence I would have been launching. Thanks for this, and for the amazing pictures. How big are the aspen borer beetles? They look huge, but it's hard to tell.

  4. Hi Kim, The beetles are about 2" long(not including the gigantic antennae). Glad you enjoyed th post.

  5. Hi David;

    I really enjoyed this post. There is a museum in Norwich Vermont named the Montshire Museum and it has a display of long horned beetles prevalent in our area. I can't remember the number now but there is something like 150 different long horns living here in New England. If you just heard the number you might say "yeh, sure." but by seeing the display you quickly can tick off the ones you have seen. Sadly to me this includes one that is doing a job on our sugar maple trees and with Vermont being the #1 producer of maple syrup, this is serious.

    The picture of the hummingbird moth caterpillar and the fact that evening primrose is a host plant is interesting. I have a small patch of evening primroses that seems to be getting bigger each year and also have a bunch of moths that similarly increases in number. They are very interesting and are often confused with actual hummingbirds by some customers. I've heard a lot of "look at the baby hummingbird" comments. Now that I know about evening primrose I'll keep an eye out. Do you have any clue to when in the cycle the caterpillars might prevail?

    As a final note, I am going to write soon about a book you might be interested in. It's titled Fruitless Fall by Rowan Jacobsen. He lives near to us and did a presentation Sunday at the community center. It's the story of colony collapse with honey bees. It was a provoking testimony to the fact that we have to learn more, learn quicker about what is going on in our environment.

    George Africa
    The Vermont Gardener

  6. Hi George,
    Thanks for the thoughtful comments. It is a horrible situation with sugar maples in Vermont. I suspect the borers are a symptom of a much larger problem- climate change. I understand the warmer, shorter winters have reduced sap production and insect outbreaks are a common next step with stress. Typically when you see borers they are targeting a tree that is already under stress, whether it is the pine beetles in the western US attacking vast expanses of trees long stressed by drought/ climate change or aspen in Missoula that are near the end of their elevation distribution.
    As far as the caterpillars, I am not sure I understand your question, about when in the cycle the caterpillar might prevail, but if you are asking about damage to the host plants, they do eat a lot of leaves, and even buds of the evening primrose, so it is best it you have a bunch of plants. They have never killed a plant though, and their caterpillar stage does not last as long at the blooming period for the primroses (at least not the ones here).
    Keep me posted on the book- it sounds really interesting.
    Thanks again for your comments.

  7. David,

    Just stumbled into your blog. I'm a bug guy myself. Mostly butterflies and odonates. We get Mourning Cloaks here as well, I have found caterpillars on our Black Willows many times. They do nectar flowers here sometimes. Thanks for this wonderful post.

  8. Rather off your main topic, but I was wondering, when I started reading this post, whether you'd be saying something about how aspen propagate. Some people (like my husband, but he's not alone) worry that planting aspen will be like inviting tree-sized weeds into the yard. A topic for another post perhaps?

  9. Those are amazing pictures of the poplar borer. I'm studying the decline of aspen in Colorado and have only seen this borer in action a couple of times. So cool.

    Yep, aspen mostly propogate by suckering underground. A lot of people don't like the development of a forest in their backyard, not to mention the aspen's vulnerability to pests. It's nice to see a tree owner appreciate the fascinating ecology of aspen.

  10. I had an infestation of borers in a clump birch that graced the window by my dining room table. It was fascinating reading about the sequence of bugs that feed on the aspen, and gave me a new perspective on the whole biological community that I was witnessing. At the time, I elected myself Protector of the Birch. You would have been amused at my determined hard water sprays on the hornets from the hose, my climbing a ladder with a homemade hornet deterrent (garlic, cayenne pepper in oil) I remember the hornets were followed by some kind of even more determined pecker deepening the holes made by the hornets. I never knew about the borers until I had a local tree guy come, and diagnose the borers, pointing out the raised trails in the bark. His prognosis was was poor, and the insecticide he used did not save the tree. I was genuinely sorry to lose the tree, and the bird community I so enjoyed observing over the seasons. I thought I'd try aspen next, not knowing (until now) that they are vulnerable to the same bugs. Perhaps having a 'live, and let live' attitude, enjoying what is there, while it is, observing and learning is less stressful, and more interesting.

  11. I was looking up places to find caterpillars in the Missoula area and found your site. My family and I have seen your house a few times and are always amazed by the beauty of your yard.

  12. Great and very interesting post. I just ran into it while looking for information on the bald-faced hornets I've been seeing on our 3 columnar poplars in the backyard. There are holes bored into the single quaking aspen in the front yard as well. It's been fascinating to watch all the activity over the last couple of years- and great to have some light cast upon some of the observations. We've actually been glad that the poplars in the backyard have been taking a hit because they were on their way to becoming somewhat unmanageable in the future. If and when you have time I'd love to hear what you think about leaving these trees as snags as they die back.

    1. Thanks for your comments! YES, leave them as snags! One of the best habitat features to attract wildlife to your garden!